AEROBIC RESPIRATION 511 



enclosing plant material with a relatively high rate of respiration in a calorim- 

 eter. Among such materials are rapidly growing stem tips, opening buds, 

 floral parts (especially during the earlier stages in their development) and 

 germinating seeds. The latter are most frequently used. To conduct the 

 demonstration two Dewar flasks or thermos bottles are partially filled, the 

 one with germinating seeds, the other with an equal mass of germinating 

 seeds which have been killed just before starting the experiment. The calorim- 

 eters are then plugged with cotton through which is inserted a thermometer, 

 and the temperature changes noted over a period of time. If the results 

 obtained are to be considered at all critical, the seeds and all parts of each 

 apparatus must be sterilized at the beginning of the determination ; otherwise 

 most of the temperature rise observed will be due to the respiration of micro- 

 organisms. Such a rise indicates the evolution of heat during the respiration 

 of such organisms, but invalidates the experiment as a demonstration of heat 

 release by the seeds. In general, in a properly set up experiment of this type, 

 the temperature within the mass of living seeds will rise to a value con- 

 siderably in excess of that recorded for the dead ones. If the dead seeds 

 have been effectively sterilized they will show little or no change in tem- 

 perature. In such experiments 100 g. of germinating seeds may release heat 

 with sufficient rapidity as to acquire temporarily a temperature as much as 

 20° C. higher than that of the dead seeds in the check experiment. In the 

 more critical experiments upon heat release by plant tissues the heat evolved 

 is expressed in terms of calories per unit of time (Table 48). 



Although the energy released in respiration is generally expressed in terms 

 of heat units {i.e. calories or kilogram-calories), not all of the energ\' is 

 evolved as heat. That portion of the energy released as heat is all lost to 

 the cells in which respiration occurs. It is pure waste as far as the plant is 

 concerned and is roughly analogous to the frictional loss of energy in a 

 machine. In warm-blooded animals, in contradistinction to plants, the heat 

 released in the respiratory process is important in maintaining the body tem- 

 perature. 



The total energy released in respiration can be calculated from determina- 

 tions of the rate of carbon dioxide evolution. If this is done for seeds in a 

 calorimeter simultaneous determinations can be made of the total energy 

 release and the energ>^ which appears as heat. The results of a series of such 

 determinations are shown in Table 48. It is evident from these figures that 

 the proportion of the total energy released as heat gradually increases during 

 germination, but in no case in this experiment did ft exceed 50 per cent of 

 the total. 



